If you want to add more weight to your horse, that requires more calories. Calories are a measure of energy, and increasing the ration's energy level is the way to promote weight gain. However, if you don't want to produce excess energy, you should add additional fat instead of high energy carbohydrates like sugars and starches. Fats that may be used include corn oil, rice bran, or other specialty high-fat products to increase the energy density of the diet. Using fats will increase weight; additionally, the energy is slowly digested and will not create a 'hot' horse.
When adding fat to the diet, it is necessary to do so slowly. For example, if feeding rice bran, 1 to 3 pounds should be adequate, depending on the size of the horse and how underweight it is. Start off with 0.25 pounds per day and slowly increase up to the desired amount over a 2 week period. Consult a veterinarian to ensure that there are no underlying health reasons for the horse's inability to grain weight, and also consult an equine nutritionist to effectively and safely add fat to the diet.
